The invention relates to a method of manufacturing a cathode ray tube, in which method a glass display panel is press-formed, reheated and subsequently reformed.
Such a method is known from U.S. Pat. No. 3,484,225. In the known method, a glass panel is press-formed, which usually takes place at very high temperatures (1000xc2x0 C.-1100xc2x0 C.), whereafter the panel is reheated above the strain point temperature of the glass and reformed in a reforming apparatus.
In this manner, a glass face panel can be precisely formed.
CRTs are becoming increasingly larger in size and weight. Furthermore, the front surface of the glass panel is becoming increasingly flatter. However, in general, an increase of the flatness of the front surface of the face panel also increases the weight of the glass panel, because the thickness of the glass panel has to be increased to ensure safety against implosion or explosion of the CRT.
Therefore, there is a great need for increasing the strength of the CRT and in particular of the glass panel.
The present invention aims at providing a method which allows an increase of the resistance of the face panel against damage and/or a reduction of the weight of the glass panel.
To this end, the method in accordance with the invention is characterized in that, prior to reforming, the glass panel is reheated to a temperature above the annealing point and, during reforming, the temperature of the glass panel is reduced to a temperature below the strain point and the face panel is not subsequently annealed.
Typically, the annealing point of glass is at approximately 525xc2x0 C. to 560xc2x0 C., the strain point being some 30xc2x0 C.-70xc2x0 C. lower. In the known method, the face panel is reformed at a temperature at or above the annealing temperature. After forming the glass panel, the glass panel is usually annealed to remove stresses.
In the method in accordance with the invention, the reforming method step takes place while the temperature of the glass panel is reduced from a first temperature above the annealing point to a second temperature below the strain point. During reforming, this reduction of temperature induces a high surface compression in the glass panel. As the glass panel is not subsequently subjected to an annealing step, as is usual, this high surface-compressive state of the glass panel is preserved. The method provides a number of advantages.
The high surface-compressive state greatly increases the resistance of the glass panel to implosion, thus increasing the safety of the CRT and/or allowing a reduction of the weight of the glass panel and the CRT as a whole. An annealing step is not necessary, and indeed unwanted, which reduces the manufacturing costs and the energy needed for manufacturing the CRT. Furthermore, although the glass panel is in a high surface-compressive state, the glass of the glass panel has a relatively low degree of compaction. During manufacture of the CRT, the glass panel is subjected to temperature and thermal expansion fluctuations. These thermal expansion fluctuations lead to stresses and deformations of the panel, which lead to a reduction of quality of the image displayed on the display screen of the CRT. The lower the degree of compaction, the less these detrimental effects occur. Therefore, by providing a glass panel having a relatively low degree of compaction, the method provides an improved image quality.
Preferably, the second temperature is at least 80xc2x0 C. below the strain point. Such a low temperature ensures that the high compressive state and the low degree of compaction are preserved and are not relaxed appreciably after reforming. In this respect, it is remarked that temperature differences may occur within the glass panel, more particularly in the innermost part of the glass panel which is at a higher temperature than the second temperature, the temperature at the surface of the glass panel.
Preferably, the first temperature is at least 30xc2x0 C. above the annealing temperature. If the first temperature is lower, the resistance of the glass panel against reforming increases, increasing the pressure needed for reforming.
Preferably, the reduction of temperature is effected within 5 minutes. If the reduction of temperature takes longer, the degree of compaction of the glass panel will increase and the surface-compressive stresses will decrease.
Preferably, the reforming step is performed in a reforming press held at or below the second temperature to bring the glass panel to the second temperature. The reforming press is preferably maintained at such a temperature, which requires less energy than in embodiments in which the temperature of the reforming press fluctuates.
Preferably, the reforming press is provided with means to increase heat transfer from the glass panel to the reforming press. The higher the rate of transfer of heat, the faster the temperature of the glass panel drops and the higher compressive stresses may be obtained in the glass panel.
A metal cloth provided between the dies of the press and the glass panel increases, for instance, the heat transfer. Such means also provide some protection for the dies of the reforming press reducing the temperature gradients in the contact layer of the die with the glass panel.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.